Mica paper



MICA PAPER George L. Gaines, Jr., Scotia, N. Y., assignor to GeneralElectric Company, a corporation of New York No Drawing. ApplicationSeptember 24, 1956 Serial No. 611,775

4 Claims. (Cl. 154--2.6)

This invention relates to the preparation of mica paper. Moreparticularly, this invention relates to sheets of mica paper which arecharacterized by improved tensile strength and to the process by whichthese improved prod ucts are obtained.

Heretofore, the preparation of mica paper has been known in the art. Forexample, mica paper preparation is described in Patents2,549,880-Bardet, 2,614,055de Senarclens, and 2,709,158Bouchet. -Micapaper prepared by the methods of these patents as well as by other knownmethods is gaining commercial acceptance because of the excellentelectrical properties of the mica paper and because of its inertness atelevated temperatures. However, one serious drawback to the universalacceptance of mica paper for many applications is the fact that the micapaper does not have as high a tensile strength as desired.

It is an object of the present invention to provide an improved micapaper.

A further object of my invention is to provide a vmica paper ofextremely high tensile strength.

These and other objects of my invention are accomplished by subjectingcomminuted mica particles to the action of barium ions in an aqueousbarium salt solution and subsequently forming a paper-like sheet fromthe resulting particles.

The term mica paper as used in this application is used in itsusual-sense to refer to a sheet-like aggregate of mica particles whichare usually heated at an elevated temperature, ground into fineparticles in an aqueous solution, and then formed into paper byconventional papermaking techniques.

The mica paper of the present invention differs from I that of the priorart in that the mica particles have been subjected to the action ofbarium ions before they are formed into the final sheet. a v

The mica paper of the present invention can be prepared by firing orheating mica, which may comprise phlogophite, lepidolite, or preferablymuscovite, at a temperature and time sufficient to partially dehydratethe mica. In general, the heating of the mica is carried out at atemperature of about 800 C., e. g., from about v 750-850 C. for a timeof about 10 minutes, e. g., from about 5 to 20 minutes. This heatingstep causes a loss in weight of the mica equal to about 2 percent byweight of the original mica. This heating has the effect of softeningthe mica while at the same time delaminating and 1 This heat increasingthe bulk volume of the mica. treated mica is then added to an aqueousmedium and agitated by any suitable device, such as a high 'speedcomminuter or mixer to convert the mica into small particles orplatelets. carried out in an aqueous suspension containing from about0.1 to 10 percent, and preferably about 1 percent by weight, of mica.This results in a pulp-like suspension of mica in which the particlesize of the mica flakes has a wide distribution. The extra-fine andextra-coarse parti cles in this comminuted aqueous suspension may thenbe The communition of the mica'can bepart of mica.

2,842,183 Patented July 8, 1958 contact with barium ions in an aqueousbarium salt solution. tact with the mica particles is not critical,andthis step may be accomplished by any desirable means. Although I donot wish to be bound by theoretical consideration, it is believed thatthe barium ions which are brought into c0ntact,With the mica particlesreplace some of the potas-. sium ions which are normally found onthe-surfaceof ,7

mica andthat the presence of the barium ions on the surface of the micatends to bind the particles of mica together with a greater force thanis found with untreated mica'containing only potassium ions on itssurface.

The barium ions can be brought into contact with the mica particles bysuspending the particles in an aqueous solution containing these ions.The formation of the suspension of mica particles in-the aqueous bariumion solution may be accomplished at any time between the time,

the mica is fired up to the time the mica is formed into paper sheets.Thus, after firing of the mica, the fired mica may be dispersedin anaqueous solution containing barium ions and comminuted while in thissolution. Alternatively, the fired mica may be comminuted in plain waterand a suitable barium salt may be added to the water to bring the bariumions into contact with the particles. As a further alternative, thefired mica particles maybe comminuted in plain water, filtered from thegrinding medium, and thenjadded to another aqueous solution containingbarium ions. -A still additional method of adding barium ions to; themica particles is to comminute the fired mica in water, separate theextra-coarse and extra-fine particles from the resulting solutionandthen either add a barium salt to the resulting slurry, or the micaparticles may be filtered from the slurry and added to another aqueoussolution containing barium ions. .In any event, the contact of thebraium ions with the surfaces of the mica particles leads to theformation of mica paper h'aving'a higher tensile strength than micapaper formed without the barium ion treatment.

Unexpectedly, I have discovered that all solutions containing bariumions are not satisfactory in the process of the present invention.Treatment of micapa rticles with a, barium hydroxide solution results inno improve ment inthe physical properties of a subsequently formedsheet. However, when a solution of barium salt is employed to treatmica, marked improvement of the physical propertiesof the resultant micapaper is observed. The particular barium salt employed is not critical,the only requirement being that the salt be soluble in water at roomtemperature (i. e. about 20 C.) to the extent ofat least about 0.5 to1.0-parts by weight of salt per parts of water. The amount of bariumsalt employed may vary over a Wide range, e. g. from about 0.1 to 10,and preferably about 1.0 part by Weight per Although 'I prefer to usebarium chloride inthe practice of my invention, other satisfactorybarium salts include, for example, barium acetate, barium nitrate,

1 mica particles and the barium ion is not-critical. Thus,

invention, the -mica particles while in comminuted form are brought intoThe method of bringing the barium ions into con,-

satisfactory for causing the replacement of some of the potassium ionsin the mica with barium ions.

After contacting the mica particleswith the barium ions, the micaparticles may be filtered from the treat+ ing solution and resuspendedin pure water.- Thispure water solution may then be used to formsheetsof paper from the mica on conventional paper-making machines.Alternatively, the suspension of mica in the barium treating solutionmay be employed directly in forming the sheets of mica paper. Regardlessof whethera pure water suspension or a barium ion containing aqueous'suspension of mica particles is used, the sheets are formed bydispersing the suspension on top of a screen and drawing the suspensionmedium through the'mica particles by means of suction. Theremaining-aqueous medium may then be removed from the mica'particles byevaporation at room temperature or at elevated temperatures. Whereelevated temperatures are employed, it is desirable to use temperaturesof from about 110-450 C. since this temperature has beenfound'satisfactory to remove all of the suspension medium from thesheet: After removal of the water, the resulting micap'articles are inthe form of a fairly rigid sheet which is extremely resistant toelevated temperatures. In addition to form= ing the final mica paper byevaporation of the suspension medium from the particles, satisfactorymica paper is obtained by calendering the damp mica sheet'resulting fromsucking the suspension medium through the mica particles on thepaper-making machinescreen. This calendaring operation may be carriedoutat a temperature of from 110-175 C. to facilitate removal of remainingmoisture from the sheet.

The mica paper of the present invention may be formed of any desiredthickness. Howeveryfor most applications it is preferred to preparesheets having a thickness of from 1 to 50 or more mils and preferablyfrom about 2 to mils. This mica paper is very useful in applicationswhich require the combination of a highdielectric strength and a goodresistance to elevated temperatures. Thus, the mica paper of the presentinvention may be employed as slot insulationfor dynamoelectric machines,as insulation in capacitors, transformers and the like, as Well asinsulation and separators in high temperature vacuum tubes.

One important advantage of the method of the present invention is thatthe barium ion solution employed to treat the mica particles may beseparated from these particles and reused to treat additional micaparticles.

The same treating solution may be used over and over again as long asadditional barium-containing material is added to the solution to keepthe barium ion concentration to whatever value is selected.

The following examples are illustrative of the practice of my inventionand are not intended for purposes of limitation. In each of the examplesa 1 percent by weight slurry of mica in water was employed. The slurrywas formed by firing Bengal Ruby muscovite at a temperature of about 800C. for about 10 minutes, at which time there had been a weight loss ofabout 2.7 percent of the original weight of the mica. After cooling,this mica was then added to distilled water and the resulting slurry wasagitated violently to comminute the mica particles. The extra-fine andextra-course particles of mica in the slurry were removed, leaving a 1'percent slurry of mica in water. All parts mentioned in the examples areby weight.

Example] A control was prepared by adding a l percent by weight slurryof mica in water to a Buchner funnel and The thickness of the controlwas about 2.3 mils.

Example 2 One part of anhydrous barium chloride was added to a 1 percentslurry of mica in water containing 1 part of mica. This bariumchloride-mica solution was allowed to stand for about 18 hours, at whichtime the mica particles were filtered from the barium chloride solution,washed several times with water, and made up into a slurry containing 1percent by weight of barium chloride treated mica. A sheet of mica-paperabout 1.8 mils thick was then prepared by the method of Example 1. Acontrol was also prepared from the 1 percent mica slurry to which nobarium chloride had been added. The bariumchloride'tre'atcdmica sheethad a tensile strength of about 7900 p.s. i. as compared with thetensile strength of about 4900 p. s. i. for the control.

Example 3 This example illustrates the treatment of mica with bariumions from the chloride and the acetate and also illustrates thetreatment of mica with barium hydroxide and shows that barium hydroxideis not a suitable treating agent for mica. All of the sheets of micapaper prepared in this example had a thickness of about 2.5 mils.

One part of barium chloride or barium acetate was added to a 1 percentslurry of mica in water containing 1 part of mica. 'In each case, thebarium-containing solution was allowed to stand for 3 hours, at whichtime a sheet was'formed-from the slurry without washing the slurry. Thetable below lists the material added to the mica slurry and the tensilestrength observed.

As the table shows, the treatment of mica particles with either bariumchloride or barium acetate markedly improves the tensile strength of theresulting mica paper. Treatment with barium hydroxide leads to noimprovement whatsoever in the tensile strength of the mica sheet.

Although the foregoing examples illustrate the treatmentof mica withonly several of the possible barium salts, it should be understood thatbarium salts not specifically illustrated may also be employed. Thus, inaddition to using barium chloride as the treating agent, mica particlesmay also be treated withbarium fluoride, barium bromide and bariumiodide to obtain the improved mica paper of the present invention.Similarly, the

mica particles may be treated with other barium salts it should beunderstood that in the practice of the pres ent invention, slurries ofmica containing 'from 0.1 to

10 percent by weight of mica may be employed. Sim- The only limitationupon the barium salt 5 ilarly, the amount of the'barium salt employed totreat the mica particles may vary from 0.1 to 10 parts per part of micain the slurry.

The mica paper prepared by the method of the present invention has thesame utility as other mica paper. Thus, this mica paper can be employedas slot insulation in dynamoelectric machines, can be cut into tape andemployed as insulation for electrical conductors, can be employed aselectrical and thermal insulation in electron tubes, etc' What I claimas new and desire to secure by Letters Patent of the United States is:

1. The method of preparing mica paper of improved tensile strength whichcomprises contacting comminuted mica particles with an aqueous bariumsalt solution,

forming a wet paper-like sheet from said particles, and drying saidsheet, said barium salt being characterized by a room temperaturesolubility of at least about 0.5 part by weight per 100 parts by weightof water.

2. The product prepared by the method of claim 1.

3. The method of claim 1 in which the barium salt is barium chloride.

4. The method of claim 1 in which the barium salt is barium acetate.

References Cited in the file of this patent UNITED STATES PATENTS2,669,764 Kilpatrick Feb. 23, 1954

1. THE METHOD OF PREPARING MICA PAPER OF IMPROVED TENSILE STRENGTH WHICHCOMPRISES CONTACTING COMMINUTED MICA PARTICLES WITH AN AQUEOUS BARIUMSALT SOLUTUON, FORMING A WET PAPER-LIKE SHEET FROM SAID PARTICLES, ANDDRYING SAID SHEET, SAID BARIUM SALT BEING CHARACTERIZED BY A ROOMTEMPERATURE SOLUBILITY OF AT LEAST ABOUT 0.5 PART BY WEIGHT PER 100PARTS BY WEIGHT OF WATER.